<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>

    <style>
        /* 
            一个盒子
            下边框顶开高，设个颜色.
            边框为透明，看不见
        */

        .box {
            width: 0;
            height: 0;
            border: 50px solid transparent;
            border-bottom: 20px solid red;
            position: relative;
            top: -50px;
        }

        /* 
        
            内容为0，是必有的
            绝对定位，给他定位过去    
        */

        .box:after {
            content: '';
            position: absolute;
            left: -50px;
            top: 20px;
            width: 0;
            height: 0;
            border: 50px solid transparent;
            border-top: 70px solid red;
        }
    </style>
</head>

<body>
    <div class="box"></div>
</body>

</html>